1. Field of the Invention
The present invention relates to a method for detecting and tracing volatile chlorinated organic compounds primarily as ground water contaminants. The invention provides a method for reacting a volatile chlorinated organic compound with CuO to form CO.sub.2 and CuCl and then reacting the CuCl with CH.sub.3 I to form CH.sub.3 Cl. The CO.sub.2 and CH.sub.3 Cl are then subjected to mass spectrometric isotope ratio analysis.
2. Background of the Invention
Chlorinated volatile organic compounds (VOCs), including the common solvents perchloroethylene, trichloroethylene, trichloroethane, and carbon tetrachloride, are among the most frequently detected groundwater contaminants in the United States. These compounds are known or suspected to be carcinogenic or mutagenic in humans. They are readily transported by groundwater, and are not reduced to acceptable concentrations for human consumption by most municipal water supply treatments; thus they represent a significant hazard to a large portion of the human population.
Measurements of the stable carbon and chlorine isotope ratios of chlorinated VOCs could be useful in tracing sources of these compounds and their breakdown products in the environment. This is particularly the case if these compounds have varied isotope ratios as a result of their manufacture and if biodegradation induces significant isotopic fractionation between residual reactants and products. It is currently difficult to determine the efficiency of natural attenuation or engineered bioremediation as a means of restoring soils and aquifers contaminated with chlorinated VOCs, but the possibility of using isotopic ratios to trace this process is now being explored. Some work has been done on measurement of stable chlorine isotopes in natural waters and rocks and recent work has been reported on a few selected chlorinated organic solvents.
Currently used analytical methods require that CO.sub.2 and CH.sub.3 Cl be prepared from a chlorinated compound by separate procedures. Van Warmerdam et al. prepared CO.sub.2 by combusting samples of some chlorinated solvents with CuO at 550.degree. C. by a published method. They then obtained CH.sub.3 Cl on separate samples of each solvent by first combusting in a Parr bomb (ASTM method D808-91) to produce chloride in carbonate solution (with yields in the range of 65-75%), and then converting the chloride to CH.sub.3 Cl, by a commonly used method of reacting AgCl with CH.sub.3 I. According to this method, the inorganic chloride is precipitated as AgCl, which is washed, filtered and dried before being sealed in a reaction tube with excess CH.sub.3 I and heated for two days at 110-125.degree. C. for reaction. The resulting CH.sub.3 Cl is separated from the CH.sub.3 I by gas chromatography before mass spectrometric determination of .sup.37 Cl/.sup.35 Cl. Other methods of converting chlorinated VOCs to inorganic chloride for conversion to CH.sub.3 Cl involve reaction with sodium metal or lithium metal.